1. Field of the Invention
The present invention relates in general to a polishing apparatus and method, and relates in particular to a polishing apparatus and method for precision polishing of a workpiece such as semiconductor wafer to a flat and mirror finish.
2. Description of the Related Art
As semiconductor devices become more highly integrated, wiring spacing becomes finer and interline spacing also becomes very narrow. Photolithographic reproduction of circuits using stepper printing processes for such highly integrated devices requires the depth of focus be very shallow so that the wafer surface must be extremely flat.
A known method of producing a flat surface on a semiconductor wafer is to press the wafer held on a holding device against a polishing tool such as, for example, a polishing table with a polishing cloth mounted thereon, and to supply a polishing solution or polishing liquid to the interface while sliding the tool relative to the wafer. This chemical-mechanical polishing process employs, in addition to a mechanical polishing process using abrasive grains, a chemical polishing process using acidic or alkaline solutions.
In such an apparatus, it is known that the removal rate of the surface material (polishing speed) is dependent not only on the force pressing the wafer against the polishing tool (polishing cloth) and sliding speed, but also the temperature of the polishing interface (polishing temperature). It is important to be able to control the polishing speed constant, not only for producing high flatness but also for determining correctly an endpoint of a polishing process.
Therefore, wafer quality control, such as flatness control within a wafer, control of variation in flatness from wafer to wafer, and endpoint determination are all related to the ability to control the polishing interface at a desired temperature. Consistency in the flatness from wafer to wafer is particularly important in improving the reproducibility of the quality of wafers produced in a serial polishing process.
Further, the polishing interface temperature is a particularly crucial element when polishing a semiconductor wafer provided with different types of thin films provided on the same surface. Such combination includes an oxide film and nitride film combination, and a combination of a metal film such as tungsten or titanium and a barrier layer such as titanium nitride or oxide.
It is anticipated that the polishing interface temperature is governed by a heat balance between heat generated by frictional forces of polishing, heat generated or removed by a polishing solution supplied to the polishing interface, and heat conduction through the wafer holding section and polishing tools. Therefore, current practices are to control the polishing solution temperature and to use a polishing table having internal cooling passages to keep the polishing table at a constant temperature.
However, it has been difficult to maintain the polishing interface temperature at a target temperature by relying only on controlling the polishing solution and polishing table temperatures of a polishing apparatus. Polishing performance at inconsistent temperatures has been a cause of wafer-to-wafer variation in thickness of films or non-uniformity of flatness within a wafer. Such a variation in wafer thickness also makes it difficult to determine an endpoint of polishing based on a target thickness.
It is an object of the present invention to provide an apparatus and a method capable of improving the uniformity of thickness within a workpiece or reducing thickness variation between serially polished workpieces, and to facilitate detection of an endpoint of polishing.
Such object has been achieved in a polishing apparatus for polishing a surface on a workpiece comprising: a polishing unit having a polishing tool such as a polishing table for providing a polishing surface and a workpiece holding device for holding the workpiece; and a polishing solution supplying device for supplying a polishing solution or polishing liquid into a polishing interface between the surface of the workpiece and the polishing surface, wherein a temperature of the polishing interface is controlled according to at least an ambient temperature of a polishing space surrounding the polishing unit, as a variable parameter.
Accordingly, the ambient temperature, for example, may be maintained at a target polishing temperature to perform precision polishing so that the polished workpiece will have less variation in thickness, thereby producing polished workpieces of a high surface quality with high yield. xe2x80x9cPolishing spacexe2x80x9d means any space surrounding the polishing unit which includes at least a polishing tool and a workpiece holding device. Such xe2x80x9cpolishing spacexe2x80x9d includes a polishing room divided by partition walls for compartmenting the polishing space from an outer clean room in a semiconductor manufacturing facility or from a storage space for storing clean wafers, or, in a case without any such partition walls, any space for installing the polishing unit. Normally, the room is provided with air supply and air exhaust devices, and a temperature control device is provided at the air supply device to control the ambient temperature.
The apparatus may be provided with an ambient temperature control device to maintain the polishing space at a specific temperature. The specific temperature is set close to the target temperature so as not to influence the polishing temperature by fluctuations in the ambient temperature.
The apparatus may be provided with a solution temperature control device to control the polishing solution or liquid at a specific temperature and a tool temperature control device to maintain a thermal medium, flowing in an internal passage in the polishing tool, at a specific temperature. These are important elements of the polishing parameters and affect the polishing temperature greatly, therefore, precise control over the polishing temperature can be provided by controlling these parameters together with the ambient temperature.
The apparatus may be provided with a sensor device to detect a polishing surface temperature. Precise control can be carried out by determining the temperature of objects to be controlled directly or indirectly.
The ambient temperature control device should maintain air temperature of the air being supplied to the polishing space in a form of a downflow. This arrangement reduces swirling up of dust particles so that the air flow is directed to enable a clean environment to be maintained.
In another aspect of the invention, a polishing apparatus for polishing a surface on a workpiece comprises: a polishing unit having a polishing tool for providing a polishing surface and a workpiece holding device for holding the workpiece; a polishing solution supplying device for supplying a polishing solution or polishing liquid into a polishing interface between the surface of the workpiece and the polishing surface; an ambient temperature sensor for directly or indirectly detecting an ambient temperature of a polishing space surrounding the polishing unit; and a polishing temperature control device for maintaining a polishing temperature at the polishing interface according to detected results of the ambient temperature sensor.
The polishing space surrounding the polishing unit is normally divided with walls and a door is provided for transferring the workpiece in and out of the polishing space.
The polishing temperature control device may control the polishing interface temperature by controlling the tool temperature or the solution temperature. The temperature of the polishing space itself may also be controlled, and a temperature detecting device for detecting a temperature of the polishing interface may be provided so that results may be used to control the polishing temperature. The detection device may be a type to detect the temperature of a polishing tool surface or a surface temperature of the workpiece being polished.
Another aspect of the invention is a method for controlling a surface temperature of a workpiece being polished by a polishing apparatus, the apparatus comprising a polishing unit having a polishing tool for providing a polishing surface and a workpiece holding device for holding the workpiece, and polishing solution supplying device for supplying a polishing solution or polishing liquid into a polishing interface between the surface of the workpiece and the polishing surface, wherein respective temperatures of the polishing solution or polishing liquid, a thermal medium flowing internally in the polishing tool and a polishing space surrounding the polishing unit are controlled so as to maintain a temperature of the polishing interface at a target polishing temperature.
The respective temperatures of the polishing solution, a thermal medium flowing internally in the polishing tool and a polishing space surrounding the polishing unit may be controlled substantially the same. These are the elements that affect the polishing temperature greatly, and by making them all the same, more precise polishing can be carried out relatively simply.
The temperatures of the polishing solution or polishing liquid, a thermal medium flowing internally in the polishing tool and a polishing space surrounding the polishing unit may be controlled according to a detected temperature of the polishing interface.
Another aspect of the invention is a method for controlling a surface temperature of a workpiece being polished by a polishing apparatus, the apparatus comprising a polishing unit having a polishing tool for providing a polishing surface and a workpiece holding device for holding the workpiece, and polishing solution supplying device for supplying a polishing solution or polishing liquid into a polishing interface between the surface of the workpiece and the polishing surface, wherein a solution temperature is controlled according to an ambient temperature in a polishing space surrounding the polishing unit and a target polishing temperature.
In the above method, a medium temperature circulating internally in the polishing tool may be controlled to maintain the polishing tool at a specific temperature.